Cellular Analysis of Gene Expression Changes in a Mouse Model of HD Altered gene expression and transcription dysregulation have been implicated in the pathogenesis of Huntington's disease (liD). Many of the alterations in mRNA abundance that have been reported in the postmortem HD brain are recapitulated in the R6/2 transgenic mouse, which expresses the N terminal fragment of the human HD gene. Gene expression profiling studies of striatal homogenates from R6/2 mice have identified changes in genes involved in signaling pathways known to be critical to striatal cell function. However, it is not known whether these changes occur in neurons, and whether all types of striatal neurons are equally affected. The striatum contains heterogeneous collection of cell-types. The majority are GABAergic and medium-sized, and it is these projection neurons that degenerate in HD whilst striatal interneurons are relatively spared. The mechanistic basis for this differential vulnerability is unknown, although it does not appear to be due to differences in the expression of I413. We propose that cell-type specific transcriptional dysregulation underlies the selective vulnerability of striatal neurons. We will apply laser capture microdissection (LCM) to harvest specific populations of chemically identified cell types from the striatum of R6/2 and wild type mice. We will examine a set of candidate genes, derived from earlier array studies, in these cell-type specific samples using quantitative real-time polymerase chain reaction. We hypothesize that transcriptional dysregulation will be most severe in the vulnerable striatal projection neurons. In contrast, we expect that that transcriptional regulation will be preserved in interneurons (expressing somatostatin/nitric oxide synthase, parvalbumin, or choline acetyl transferase), which are resistant to the disease process. We further hypothesize that the mRNA abnormalities in the R6/2 mice will be attenuated by chronic 2% creatine dietary supplementation, a treatment regime previously shown to rescue striatal cell atrophy in R6/2 mice as well as retard the HD phenotype. To achieve the cellular resolution required for this study we will develop specific rapid immunohistochemical protocols that are compatible with LCM to facilitate the harvesting of mRNA from specific chemically identified cell populations. Understanding transcriptional dysregulation at a cellular level may facilitate the search for novel therapeutics for HD and other polyglutamine diseases.